In experiments using tissue culture methods to grow (newborn) mice calvaria in association with ellicited peritoneal macrophages (isolated from adult mice) a 23 percent decrease of calcium in the calvaria was observed. This response was measured after 7 days in culture. Scanning and transmission electron microscopy revealed many macrophages attached to the surface of the bone matrix. They were closely associated with the matrix surface but did not resemble osteoclasts, except for occasional multinucleated cells. It is concluded that macrophages resorb bone in this system but that it is different from osteoclastic bone resorption.